Generally, a polishing material includes abrasive grains and a binder. Several kinds of polishing materials can be manufactured by controlling the kinds or ratio of those elements in accordance with particular polishing requirements and, if necessary, by adding a filler or a reinforcement.
Natural abrasive grains have been in use heretofore, but, artificial abrasive grains, which have advantages in hardness, strength and thermal resistance have been used frequently more recently.
Vitrified inorganic binders and organic binders have been used with abrasive materials. Phenolic resins are popular organic binders.
Vitrified inorganic binders have the disadvantages that they are glassy and fragile. Phenolic resins are hard for an organic binder, but are also fragile and are likely to be broken down by the vibration while polishing as a result of the lack of bonding strength.
In order to overcome these disadvantages, an abrasive material, containing polyurethane resin as a binder, has been proposed lately, as described in U.S. Pat. No. 3,770,555. Since polyurethane resin has a low softening point (about 140.degree. C.), it cannot be used except under such conditions that the temperature is controlled below its softening point, for example, in water.
A synthetic resin that is excellent in thermaal resistance is not always suitable as a binder for abrasive material. In order to determine what resin is suitable, the following properties should be evaluated: abrasive grain binding, friction resistance (lubricity resulting from the self-wearing of the binder while polishing) and self-sharpening.